1. Field of the Invention
This invention relates generally to techniques for detecting flux transitions from magnetic media, typically for the recovery of magnetically recordable digital information from such media and more particularly relates to improvements in qualifying techniques for enhancing the accuracy and reliability of the recovery of magnetically recorded digital information.
2. Description of the Prior Art
Digital data is conventionally stored on magnetic media in the form of flux transitions on the surface of the media. Data recovery consists of determining the presence, and relative timing, of such flux transitions. As the density of the storage of digital information on magnetic surfaces continues to increase, it has become more difficult to accurately and reliably determine the presence or absence of such flux transitions because of the decreasing magnitude and quality of the analog readout waveform.
The spacing of the flux transitions encodes the data being recorded. The read head detects these transitions to provide positive and negative pulses corresponding to the flux transitions on the media, that is, the sudden reversals of the magnetic field. The signals produced by the read head in response to such transitions are low amplitude and include noise and/or other unwanted signals. Such read head signals are therefore amplified and filtered before peak detection. The filtering is intended to enhance the quality of the signals, representing the flux transitions to be detected, and attenuate the unwanted noise.
In the ideal system, a Read Data Pulse (RDP) would then be detected in the amplified and filtered output of the read head for each actual flux transition in the media. As the requirement for higher density recording formats continues to increase, however, it has become more difficult to faithfully distinguish between genuine RDP's, representing actual flux transitions in the media, and bogus or spurious RDP's which result from other causes, such as noise.
One improvement in flux transition detection has been the development of pulse amplitude qualifying circuits to aid in distinguishing genuine from spurious RDP's based on amplitude. In conventional pulse amplitude qualifying systems, the analog signal from the read head is applied to a peak detector to produce a signal including genuine RDPs. The peak detector output also may contain spurious RDPs resulting from noise or other unwanted signals.
In addition, the analog signal from the read head is rectified to produce a slowly varying DC voltage level proportional to the peaks being detected. The proportionality is typically controlled by a level set circuit at about 50% of the peak amplitude. The level set circuit includes a time constant circuit which permits the DC voltage level to change slowly in accordance with changes in the genuine peak amplitudes in the analog signal due, for example, to a loss in amplitude of the analog signal.
The decay rate is typically about 10% per minimum time interval, that is, about 10% of the time interval between two flux transitions in the magnetic media spaced as closely as permitted by the particular coding scheme being used. The decay rate permits the peak detector to operate properly even during dropouts when the analog signal drops to a lower level. Such dropouts often last for 50 to 200 microseconds and may reduce the analog signal amplitude by as much as 25% or more.
The slowly varying DC voltage level is applied to a hysteresis flip flop to create positive and negative threshold values. The analog signal from the read head is applied to the hysteresis flip flop to produce an output level change whenever the analog read head signal exceeds the appropriate positive or negative threshold value. The output level change is then applied to the signal input of a clocked flip flop while the peak detector output provides the clock pulse so that the timing of the output of the clocked flip flop is controlled by the detected peaks, but qualified by the output of the hysteresis flip flop. The output of the clocked flip flop may then be applied to a bi-directional one shot to produce a conditioned pulse representing genuine RDPs.
In other words, in a conventional amplitude pulse qualifying circuit, a DC voltage set to a controllable proportion of the recent amplitude of the peaks in the analog read head signal is used as a threshold value to distinguish between genuine and spurious RDPs.
However, certain patterns in the analog read head signal are more difficult than others to read and/or distinguish genuine from spurious RDPs. A pattern of flux transitions may conveniently be described as a series of 1's and 0's where the 1's, for example, represent transitions on the media and the 0's represent no transitions on the media at a place on the media where a genuine transition could occur. In this example, a 1 represents an RDP while a 0 represents a lack of RDP. In conventional systems, RDPs may genuinely occur at time intervals which are integer multiples of a basic time interval known as one cell time.
One pattern which tends to cause difficulties in accurate detection is known as the isolated tri-bit pattern in which three 1's occur spaced as closely together as the encoding scheme permits. In the analog signal, the first and third 1 would have a first polarity while the second 1 would have the opposite polarity. The resultant analog signal is the summation of these opposite polarity pulses. The central pulse is often substantially attenuated and therefore often missed, or rejected, by conventional amplitude qualifying techniques. The problem of missed central pulses in an isolated tri-bit pattern is aggravated by dropouts as well as low frequency noise which can appear as a baseline shift, that is, a change in the average level of the analog signal peaks.
Conventional approaches to reducing the isolated tri-bit pattern problem in amplitude pulse qualifying techniques include reducing the level of the DC voltage level applied to the hysteresis flip flop by reducing the proportion of the peak amplitude used to define the level and/or the use of slimming filters. Slimming filters, often used in such systems, narrow the peak widths, reducing the effect of the summation of the isolated tri-bit pattern on the amplitude of the central pulse.
Another pattern which tends to cause similar difficulties in accurate detection is known as the low frequency problem in which isolated 1's are separated by many 0's. Such patterns would ideally have a flat area between the isolated 1's, but such flat areas are vulnerable to noise. One conventional technique for reducing the low frequency problem is to change the filter characteristics to fatten the pulses, that is, reducing any slimming filter effects, and/or to increase the hysteresis threshold.
In conventional systems, the filter and hysteresis level settings are therefore often a compromise between minimizing the isolated tri-bit and the low frequency problems. The combined problem is therefore often called the tri-bit low frequency problem.
In addition, magneto-resistive (MR) read heads are replacing conventional inductive read heads. MR heads, however, are more susceptible to the generation of low frequency noise which is difficult to attenuate by filtering. MR heads therefore accentuate difficulties in accurately recovering data, such as those caused by pattern sensitive problems like the tri-bit/low frequency problem.
What are needed therefore are data recovery enhancements providing more reliable qualifications schemes, especially for use with MR heads.